I have a fountain at our church that was built from scratch by an artist and I need some control circuitry. One of our members started on the project but his health has deteriorated and he cannot find his schematics. I have some experience with electronics. I used to build TTL logic circuits with gate chips, flip-flops, etc. when Radio Shack still had tube testers. Nothing since, thus I am very rusty. So-

Requirements 
Sense the water level to stop the fountain pump if the water level is too low.
Automatically control a 24vac irrigation valve to fill the reservoir.

I thought about mechanical at first but here are the issues.

1) Fountain is set in concrete pad [part of the design]. Reservoir is under fountain, below grade and not visible.
2) Fill/drain plumbing is already in and is under the concrete & fountain. The irrigation valve is also in place on the edge of the pad - about 9ft [2.75m] away.
3) Access to reservoir and pump maintenance/removal is under several inches of stone. It is a cylinder about 8in [20cm] dia. x 10in [25cm] deep. Clearance is about 1 to 2 in [2.54 to 5cm] total on the sides when removing pump.

Could you please eyeball the attached? As I stated, I am very rusty on my circuit design - especially non-ttl. I got the idea to use a Schmitt NAND gate from a plant soil moisture indicator [preventing dc electrolysis of probes]. So I 'hijacked' the concept and expanded with some digital logic concepts. You will see that I am using U1B as an oscillator to generate an ac signal for all of the probes. My main concern is if I am using the correct logic - positive vs negative.

Also - I would like to include LEDs to indicate which probe is above water level. I can borrow ideas from other schematics with npn transistors driving the relay coil but I have forgotten how with pnp transistors.

There's a lot there I don't know about in that diagram but one thing I'm certain of is that you should use MOSFETs instead of those transistors. The MOSFETs would not need a base current, and if properly controlled by the voltage at their gate, will behave just like a switch; very low resistance when on and very high when off. You just need to be sure to have enough voltage at the gate to turn it fully on, 10-15V for a regular FET and <5V for a logic-level FET.

Just had a look at the pump part. The output relay seems to open when the resistance between signal and pump probe gets higher than 8M and will turn on again when resistance is lower than 2M.

I'm sure that for the 2M part this is going to work (enough water in the fountain). But for the 8M I'm not sure. How far are these probes away from each other? Can you estimate what will be the resistance once there is still water there, just not enough, but the probes are still wet etc..?

Also, do I have to have two circuits? I mean either the water level is too low, then I turn on the irrigation valve. Or it's ok then I turn on the fountain pump. They are exclusive, aren't they?

why make it any more complicated? when the water drops below the level of the wire, pump will turn off and valve will turn on, and vise versa.

If you're worried about relay chatter when the water level is fluctuating around the level where the wire barely touches the surface, add a second wire for the solenoid a little bit higher than the first so that the solenoid will stay on for a little while after the pump kicks on.

If you're worried about the resistivity of the water being too high to trigger the relay, but the depth sensing wire very close to the ground and dump a bag of salt in the fountain.

why make it any more complicated? when the water drops below the level of the wire, pump will turn off and valve will turn on, and vise versa.

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Is this a block diagram, strantor? I just measured the resistance of the water I had in my breakfast drinking glass It has 200k if the probes are at a 1cm distance. What relay could I turn on with 200k resistor in series?

I just measured the resistance of the water I had in my breakfast drinking glass It has 200k if the probes are at a 1cm distance. What relay could I turn on with 200k resistor in series?

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Yes this whole bit about the high resistivity of water gets confusing. I had another topic on it a while back (one of my first posts, actually). Your meter measures 200k across a glass of water using 1V (I assume). My meter measures 14KΩ across my glass of water with 1 V, meaning my water is less pure than yours. Are you drinking purified water? I doubt the outdoor water fountain has purified water in it. It's resistance is probably lower still than my glass of water.

now, What I have found is that water presents lower resistance to higher voltages. I would call it "Strantor's law of water resistivity inverse proportionality" but I'm pretty sure someone's already claimed it. For example, my glass reads 14KΩ using 1V. When I hook up a 12V battery to the water and read the current it draws, I get 7ma. Using Ohms law, that works out to 1.7KΩ, far less than 14KΩ. Now I add a teaspoon of salt to the glass and I read 6KΩ with my meter. dunk the battery leads and I get 240mA, which works out to 50Ω. I connected my relay up to the battery with the semi salty water and it works just fine (it didn't work until I put the salt in).

Now I don't think you are going to want sea water in your fountain, but I also think you have a better chance of success using 24vac than 12vdc and you might not even need to add salt. Especially considering its outside and you get rainwater in there and all kinds of pollution.

Yes this whole bit about the high resistivity of water gets confusing.

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That's because you're not just measuring the intrinsic property of the water, you're measuring that PLUS the extrinsic properties of the surfaces you're exposing to the water, the probe tips, wire strands, whatever. Add fouling, temperature dependance, bubble formation, and so on, and you have a really chaotic and unpredictable mess. Welcome to electrochemistry.

That's because you're not just measuring the intrinsic property of the water, you're measuring that PLUS the extrinsic properties of the surfaces you're exposing to the water, the probe tips, wire strands, whatever. Add fouling, temperature dependance, bubble formation, and so on, and you have a really chaotic and unpredictable mess. Welcome to electrochemistry.

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yes lots of variables. I don't fully understand any of it, but the idea is that it is possible to energize a relay using water as a conductor.

Now I don't think you are going to want sea water in your fountain, but I also think you have a better chance of success using 24vac than 12vdc and you might not even need to add salt. Especially considering its outside and you get rainwater in there and all kinds of pollution.

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Ok, it might work, but it may not be stable enough to be a reliable option. Also, if the waters resistance is fluctuating too much, wouldn't I possibly overstress the relay, i.e. applying too much voltage on it?

I think the circuit the OP proposed is not bad, but until now I don't see the need for two separated circuits, if their functions are exclusive.

To clarify the reasoning for my design using more of a layman terminology vs. technical so I can explain without the possibility of 'misspeaking' using an incorrect term. Normally you think signal to ground but to explain, I am going to reverse the process.

1) U1B, C3 & R1 is used to create an AC signal. [See attached from TI's CD4903B data sheet - nand_astable.gif]. C1 blocks any DC flow. This probe is also used as the common or 'ground' for the water fill probes. C2, C5 & C7 also blocks DC on the other end.
2) D2, D3 & C4 rectify the signal and because the Schmitt NAND gate is very forgiving on the trigger voltage to change logic states, U1A output can now be used as a DC source for energizing the relay.
3) When the water level rises above the pump probe, this completes the circuit and energizes the relay  thus the fountain pump creates the flowing water.
4) U2A & U2C do the same for the valve. Adding U2B creates a pseudo flip-flop with the logic shown in logic.gif [0 being above water or dry, 1 being in the water].

To answer some general concerns:

90%+ of our water comes from Lake Mead [aka Hoover Dam] and due to the geology is highly mineralized.
The levels of the probes are:
Ground [signal]  bottom of reservoir
Pump  just above the 'pumping air' level
Fill  above the pump safety probe
Close  at the full reservoir level

wayneh  Never worked with a MOSFET. Not opposed to the idea just need more details.

simo_x  PNP in this configuration was what was used in the original circuit. I was confused myself because I have seen several 555 based schematics and they used NPN. Unless this is because it is a 'ground' generated signal versus the 'positive' logic gate signal used with the 555. This is where I forgot almost everything and need the most help. About the only thing I can do is tell the difference between NPN & PNP and that there needs to be some-sort-of biased voltage somewhere to trigger the transistor.

praondevou  I think of this as one circuit. I just drew it in function blocks. Maybe I should have moved the ground / signal probe into a separate block keeping pump detection probe & water level probes as blocks.

90%+ of our water comes from Lake Mead [aka Hoover Dam] and due to the geology is highly mineralized.

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This was also my concern when I asked about the resistance. I put the circuit into the simulator and it seems that if the resistance gets higher than 8Mohm the pump will turn off.
Question was: Minerals will deposit at the fountain walls, when the water level lowers. Will the resistance between signal and pump probe get higher than 8MOhm without water?[/QUOTE]

Ok, it might work, but it may not be stable enough to be a reliable option.

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Yeah you're probably right. I've never used water as a conductor over any length of time. It might work one day and not work the next day depending on whats in the water; I really couldn't really say. In short term tests, this has always worked for me

Also, if the waters resistance is fluctuating too much, wouldn't I possibly overstress the relay, i.e. applying too much voltage on it?

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with the water acting as a series resistance, I don't see how it could amplify voltage. Just to be clear I was suggesting using a 24Vac source and a 24Vac relay. I did my experiment with 12VDC because I didn't have a 24vac transformer handy.

I think the circuit the OP proposed is not bad, but until now I don't see the need for two separated circuits, if their functions are exclusive.

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I personally don't understand Lynx's circuit because I'm not as electronically savvy as you guys. That being said, I posted my idea because I don't see the need for all this circuitry for something so simple. seems overcomplicated/overengineered to me. There's a saying that the best system is the simplest system that works.

The fact that lynx didn't even address my post is a good indicator that it isn't being considered, so this is where I leave off; I have nothing more to add.

with the water acting as a series resistance, I don't see how it could amplify voltage.

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Ok. I thought to overcome the resistance I'd need a higher voltage than the relays rating. If then the resistance lowers I could overstress the relay. that was my idea. kind of a misunderstanding.

@LINX20

I think the output logic of the U2C is inverted. At step 3 and 4 of your truth table the output of U2C will be LOW, so the PNP conducts, the relay it turns on and the valve too. But these are the cases where the valve should be off (blocked), right? Because the water level is above either the FILL or the CLOSE probe.
Correct me if I'm wrong.

You could use an npn instead of the pnp. Don't forget to put the base resistor.

Strantor  MY APOLOGIES! Just joined and am getting used to the etiquette of this board. I respect all opinions. Even those that ask me to take a long walk off a short pier. I did not mean to leave you out. An electronic circuit reduces the risk of too much current in the water. With 2 young'ins that love to walk up to the fountain and put their hands in the flowing water; safety is a number one concern.

The concept of resistance on the probes is when the signal probe [as I have been also calling ground, since many DC circuits call the common probe ground] & the sensing probe are underwater  resistance is low. Dry or above water level resistance is high. Thus giving a 1 [wet] or 0 [dry] to the inputs of U1A. Being a NAND with inputs tied together, the output [pin 3] is inverted.

This is where I am lost. Do I select a transistor for sourcing current or sinking current and where is this located relative to the gate output pin. Biasing requirements for the transistors is beyond what I remember from basic electronics. [I had a section on vacuum tubes in my last formal class] This is also my problem of how to interface a transistor with the logic part. I could insert U2D to invert the output of U2C for the correct transistor selection.

Maybe my question is what transistor and resistors do I need with a 0 logic to energize the relay vs. a 1 logic to energize the relay?

This is why I am attempting AC probes  to minimize the electrolysis that is caused by current flowing. I suspect DC would kill even stainless steel probes within a few months. The ions in the water are an advantage though when wanting low resistance water.

The concept of resistance on the probes is when the signal probe [as I have been also calling ground, since many DC circuits call the common probe ground] & the sensing probe are underwater – resistance is low. Dry or above water level resistance is high. Thus giving a 1 [wet] or 0 [dry] to the inputs of U1A. Being a NAND with inputs tied together, the output [pin 3] is inverted.

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This is all clear. The question was what is the resistance when it's dry? It's a few megaohms, how many? More than 8MOhm?

This is also my problem of how to interface a transistor with the logic part. I could insert U2D to invert the output of U2C for the correct transistor selection.

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The pump part could work the way you drawed it.
Do you agree that the logic is inverted for the irrigation part? You can either invert the signal with the spare gate (as you said) and use Q2 the way it is or you replace Q2 with an NPN transistor, if the collector current (current through the relay) is known you can then determine the necessary base current.

Sounds like a fun project. I've been making solar powered fountains and pond controllers for a couple of years. I have added all the auto fill and don't run if no water logic too. I used a microcontroller because it's easier for me to solve hysteresis issues, and others, in code rather than analogue electronics. You are welcome to my designs and PCB's if you're interested let me know and I'll send you the links. http://backyardsolar.blogspot.com/